The rzr's are basically a non-street legal jeep at this point. They are just 3" short of being the same wheelbase as a CJ-2A, and I think about the same power!
In WA, I would lean towards a dual sport, even if you trailer it to your riding area. You can at least ride on roads AND trails and not get in trouble.
Not much for quads in this state unless you like mud, or fighting with jeeps to share the trails.
rzr's your pretty much limited to jeep trails and sand dunes.
Sure there is Moses lake sand dunes (moses hole ash pit), and Beverly dunes (another ash pile near moses lake), but they are pretty poor excuses for sand. For about the same amount of driving from the Tacoma area, you could be at Sand Lake on the Oregon coast and get a sample of what dunes can really be like. If you get hooked, then head south for the bigger oregon dunes.
A quad will go places a rzr cant, and depending on size of quad and rider skill, do it faster too. The bike will go places neither can go, but you gotta work for it. And sand sucks on a bike unless your into that kinda pain. The rzr can haul a passenger comfortably, and you might get a better half to want to go riding more.
Your really comparing apples to oranges tho. You need to pick the places you like or want to ride, and then pick the vehicle, and if you want a passenger or not.